Furnace Maintenance: Checked operation of thermostat. Changed 2(10x20x1) return air filters. Checked for gas and co leaks. Pulled and polished flame sensor. CHECKED HOT SURFACE IGNITER AND FOUND BURNT OFF SILICONE AND WHITING. Cleared pressure ports and tubing. CHECKED CAPACITOR R:7.5 A:4.7mfd. Checked amp draws on inducer and blower motor. CHECKED COMBUSTION 12.4% OXYGEN AND 18 PPM. Static total was .52" w.c. Temp rise 55 degrees Replaced failing hot surface igniter and 7.5mfd capacitor and combustion after balance was 8.9% oxygen and 47 ppm.